There's Absolutely No Evidence That Pesticides Are To Blame For The Rise In Microcephaly

The larvicide pyriproxyfen has now been accused of causing microcephaly. Jung_Rattanasiri

A group this week has announced that the thousands of cases of microcephaly reported this year aren’t down to the Zika virus, but are instead due to pesticides.

The report, compiled by an Argentinian organisation called Physicians in the Crop-Sprayed Villages, highlights the fact that some areas hit hard by the epidemic have not reported any cases of microcephaly. For instance, more than 5,000 pregnant women have been infected in Colombia, but there has been an apparent absence of this birth defect, which causes babies to be born with abnormally small heads. Yet in Brazil, there have been more than 460 cases confirmed since the outbreak came under investigation last year, and more than 3,800 suspected cases.

The link between microcephaly and Zika has yet to be confirmed, but is strongly suspected by health professionals. The report however proposes an alternative explanation: a larvicide called pyriproxyfen, which has been added to drinking water supplies in some regions of Brazil since 2014 with the aim of reducing mosquito numbers. And of course, just to make the story sound more convincing, the document points out that pyriproxyfen is produced by a subsidiary of Monsanto, Sumimoto Chemical. However, Monsanto has been quick to point out to Tech Times that it does not own the company.

Some afflicted countries have advised women to delay becoming pregnant because of the Zika epidemic. Mario Tama/Getty

Regardless of the relationship with the much-hated biotech giant, there are scientific reasons as to why this theory isn’t plausible. Pyriproxyfen works by mimicking a hormone present in numerous insect pests, which has the effect of interfering with the process of metamorphosis and preventing them from reaching reproductive stages. As Dr. Ian Musgrave from the University of Adelaide points out to the Science Media Centre, this hormone isn’t present in vertebrates and thus has extremely low toxicity to mammals.

“In terms of how much is present in water reservoirs that have been sprayed with pyriproxyfen to control mosquito larvae, a person would have to drink well over 1,000 litres of water a day, every day, to achieve the threshold toxicity levels seen in animals,” he adds.

A number of studies have been conducted in several mammals, exploring both the potential short- and long-term effects of pyriproxyfen exposure, and none of them identified evidence of developmental problems or neurotoxicity. In contrast, the Zika virus has now been identified in the brains of babies and a fetus with microcephaly, and is also linked with another neurological condition called Guillain-Barré. And while it is difficult to prove cause and effect, the overwhelming evidence points to Zika as the likely driver behind this surge in microcephaly cases.

Alongside being incredibly frustrating to researchers and public health experts, unscientific reports such as this have the potential to be extremely damaging. If this pesticide has the potential to safely reduce the burden of mosquitoes, which are carriers of more than just Zika, then it can also reduce the burden of disease.

As Professor Andrew Batholomaeus from the University of Queensland stresses: “The potential human health consequences of discouraging the use of pyriproxyfen in drinking water storage and other mosquito-reduction programs is catastrophic with potential deaths and serious disease from otherwise avoidable malaria, dengue and other mosquito-borne diseases numbered in at least the hundreds of thousands. If these reports and suggestions are motivated by anything other than ignorance and poor scholarship they are deserving of the most strident condemnation.”